Experimental study on fractal characteristics of fault filling medium in the tunnel and relationship between fractal dimension and permeability coefficient

Water and mud inrush due to particle migration of filling medium in fault results in casualties, equipment damage, project delays, and other serious consequences. The particle size distribution of the filling medium plays an important role in the occurrence of water and mud inrush and affects the permeability coefficient. To study the influence of the particle size distribution on the permeability coefficient, more than 120 cases of fault filling medium in the tunnel were analyzed. Fractural dimension can more accurately reflect the changes in the content of particle fraction of the filling medium, so the fractal theory was introduced to calculate the fractal dimension, it was found that the filling medium had one or more fractal dimensions, and most of them only had a fractal dimension. The fractal dimension of the sample with one fractal dimension varied from 2.335 to 2.950. The permeability coefficients were obtained by the self-designed integrated penetration apparatus with constant and variable heads. The results showed that the relationship between the permeability coefficient and fractal dimension was power function. The permeability coefficient of the filling medium decreased with the increase of the dry density in each fractal dimension. Finally, according to the grain size composition, fractal characteristics, change law of permeability coefficient and real cases of water inrush, the filling medium of fault was divided into broken filling medium, loose filling medium, and dense filling medium. The water inrush caused by loose filling medium and dense filling medium had obvious time lag. Geological prospecting should be strengthened in the construction to find the disaster source in time. These studies in this paper provide an important basis for the safe and rapid construction of tunnel engineering.